The present invention relates to an integrated planar waveguide optical device suitable for switching optical signals between three or more ports and/or modulation of the optical intensity, and more particularly, to an integrated optical Mach-Zehnder interferometer device having low polarization dependent loss at a high level of attenuation or signal isolation.
Integrated optical switching or modulation devices are based on the conventional Mach-Zehnder (M-Z) interferometer geometry and comprise two or more channel waveguides formed on a planar substrate. The two channel waveguides are usually parallel to each other and separated at the terminal ends, which form the input and output ports of the device. The waveguides come in sufficiently close proximity to each other in two discrete regions permitting exchange of light by optical coupling. These regions, referred to as optical couplers, generally divide the incident light beam arriving from an input port of one waveguide equally between both waveguides. Thus, light entering one waveguide is split in the first coupling region, or input coupler, so that it propagates with equal intensity in both waveguides until it reaches the second, or output, coupler. Accordingly, the portion of the channel waveguides located between the input and output couplers are referred to as the waveguide arms. If the optical path length of both arms is the same in the normal, that is un-activated state of the device, the two beams recombine in phase and are transmitted without modulation of intensity to a common output port, the incident optical signal crossing from the first channel waveguide input port to the output port of the second channel waveguide.
If the optical path length of the two arms is different by a half wavelength, which is out of phase, the two beams recombine without modulation of intensity at the output port associated with the same waveguide input port.
Thus switching of signals between output ports is achieved by the selective modification of the optical path lengths of waveguide arms by a half wavelength. Selective control of the optical path length differences of less than half a wavelength split the incident beam energy between the output ports, permitting the device to be used as a variable optical attenuator as well as an optical switch.
As the optical path length of the waveguide arms is a function of the local refractive index in the waveguide media, modulation of the refractive index of either of the waveguides changes the optical path length to cause either partial or total destructive interference of the co-propagating optical signals, which permits the selected attenuation of the output signal, or switching of the signal to the output port of the first waveguide.
Refractive index modulation in one or more arms of the M-Z optical waveguide device may be accomplished by exploiting the electric or magnetic field responsive optical properties of particular waveguide materials, or by utilizing the thermo-optical properties or stress-optical properties of a wide range of materials. An actuator is connected to a control circuit such that the selective application of an electromagnetic field or bias to an actuator associated with one or more arms of the device induces a strain, temperature change or otherwise modifies the refractive index of the selected arm, or arms, to induce the desired phase difference. For example, U.S. Pat. No. 5,502,781, which is incorporated herein by reference, discloses integrated optical devices which utilize either a magnetostrictively, electrostrictively or photostrictively induced stress to alter the optical properties in one or more waveguide segments of the device. Latchable integrated optical devices are achieved by utilizing a controlled induced stress to xe2x80x9ctunexe2x80x9d one or more waveguides to a desired refractive index or birefringence, which will be retained after the field is removed.
Thin film heaters are a preferred actuator for exploiting the thermo-optical properties of the waveguide materials, being generally compatible with other thin film processes and materials used to fabricate the waveguide and/or substrate.
However, it has been found that thin film heaters and other actuators limit device performance via second order effects. Channel waveguides are generally fabricated from materials that are homogenous and optically isotropic in the bulk state, having a single refractive index. Device fabrication methods and actuator designs may induce optical anisotropy in the channel waveguide such that the refractive index will vary depending on the polarization state of the incident light propagating in the waveguide. The difference in refractive index of a material is referred to as birefringence. The propagation characteristic of unpolarized light in a birefringent media is readily evaluated by decomposition into vectors of orthogonal polarization states, TM and TE. This difference in birefringence between the two arms of the M-Z interferometer results in a polarization dependent loss in the optical signal.
Prior art integrated planar waveguide M-Z devices, such as disclosed in WO 00/52518, which is incorporated herein by reference, suggest that the actuation mechanism should be designed so as to avoid introducing birefringence in the plane orthogonal to the direction of signal propagation in either of the waveguide arm segment. More specifically, this application discloses a method of placing piezoelectric ribs actuators on selected region of a Mach-Zehnder optical device to minimize differential strain perpendicular to the waveguide channel.
Accordingly, it is an object of the present invention to provide an integrated optical Mach-Zehnder interferometer device having a low polarization dependent loss at high levels of signal attenuation or isolation.
It is a further object of the invention to provide a simple means for decreasing the polarization dependent loss that avoids the addition of compensating components or additional process steps in fabricating the integrated optical Mach-Zehnder interferometer device.
An integrated optical Mach-Zehnder interferometer device comprises a first and second channel waveguide formed in or on a substrate which are connected at their terminal ends to plurality of input and output ports through an input coupler and an output coupler.
In the case of thermal optical switches or variable attenuators where the selective adjustment of refractive index of the waveguide channel material occurs by selective temperature change of one of the channel waveguides, the polarization dependent loss (PDL) may be significant depending on the thermal properties of the substrate and the temperature change required to sufficiently modulate the refractive index of the optical media forming the channel waveguide. Moreover, it has been found that polarization dependent loss may increase dramatically as the incident optical signal is attenuated or switched, as even a linearly proportional increase in birefringence with refractive index of the channel waveguide material results in a non-linear increase in PDL.
In one aspect of the invention the aforementioned limitation is overcome by utilizing a first or second channel waveguides having an initial birefringence xcex94, to compensate for an increased birefringence on actuation of the switch or modulation device. The first and second channel waveguides are preferably of unequal physical path length such that the optical path length is substantially equal when the device is not energized. The change in birefringence induced by heating the first or second channel waveguide is of equal magnitude to the initial birefringence in the first or second channel waveguide.